1. Field
The disclosure relates generally to the field of interference cancellation systems and methods, and, in particular, to systems and methods for selectively cancelling interference using a diversity receiver chain.
2. Background
Advanced wireless devices have multiple radios (e.g., WWAN, WLAN, WPAN, GPS/GLONASS, etc.) that operating on the same, adjacent, or harmonic/sub-harmonic frequencies. Various combinations of radios cause co-existence issues due to the relative frequencies. In particular, when one radio is actively transmitting at or close to the same frequency and at a same time that another radio is receiving, the transmitting radio can cause interference to the receiving radio. For example, same band interference may occur between Bluetooth (WPAN) and 2.4 GHz WiFi (WLAN); adjacent band interference between WLAN and LTE band 7, 40, 41; harmonic/sub-harmonic interference may occur between 5.7 GHz ISM and 1.9 GHz PCS; and an intermodulation issue may occur between 7xx MHz and a GPS receiver).
The interference cancellation requirements increase due to increasing level of interference from both in-device and inter-device radios. This can result in severe degradation of the receiver performance. There are many proposed interference mitigation techniques in time, frequency, spatial, software, or power domains. However, most of these techniques relate to interference avoidance/coordination that do not fully enable concurrent transmission and reception. This results in the degradation of spectral efficiency.
Active interference cancellation (AIC) cancels interference between a transmitter radio and a receiver radio by matching gain and phase of a wireless coupling path signal (hc) and in a wired AIC path.
The AIC can happen in RF (radio frequency), BB (baseband), or both RF/BB. AIC in BB only shows limited cancellation performance because the coupling path signal is much stronger than the desired signal strength (i.e., interference level is much higher than desired signal level), easily resulting in saturation of RF components, such as LNA (low-noise amplifier) and ADC (analog-to-digital converter), the limiting the applicability of the BB technique.
AIC in RF provides better cancellation performance. In prior art RF AIC techniques, the transmit signal is coupled using a coupling device and then subtracted at the receiver after adjusting the gain equal to the signal received from the receiver. This approach has some limitations. First, it is hard to match the group delay between the AIC path and the coupling path. Second, transmitter noise may fall into the receiver band, causing significant increase in the noise figure after the LNA. Third, the AIC cannot be turned off regardless of the strength of the interference. Fourth, a small but non-negligible coupler is needed in the transmit path to provide a reference signal to the AIC path.